netdev - Re: [PATCH net] 6lowpan: add missing fragment list spinlock

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Message-ID: <20140205074729.GA27973@omega>
Date:	Wed, 5 Feb 2014 08:47:30 +0100
From:	Alexander Aring <alex.aring@...il.com>
To:	David Miller <davem@...emloft.net>
Cc:	alex.bluesman.smirnov@...il.com, dbaryshkov@...il.com,
	linux-zigbee-devel@...ts.sourceforge.net, netdev@...r.kernel.org,
	werner@...esberger.net
Subject: Re: [PATCH net] 6lowpan: add missing fragment list spinlock

Hi David,

thanks for your reply.

On Tue, Feb 04, 2014 at 08:32:03PM -0800, David Miller wrote:
> From: Alexander Aring <alex.aring@...il.com>
> Date: Tue,  4 Feb 2014 11:57:53 +0100
> 
> > @@ -197,7 +197,9 @@ static void lowpan_fragment_timer_expired(unsigned long entry_addr)
> >  
> >  	pr_debug("timer expired for frame with tag %d\n", entry->tag);
> >  
> > +	spin_lock_bh(&flist_lock);
> >  	list_del(&entry->list);
> > +	spin_unlock_bh(&flist_lock);
> >  	dev_kfree_skb(entry->skb);
> >  	kfree(entry);
> >  }
> 
> This will deadlock, because the other code path holding flist_lock calls
> del_timer_sync() to wait for this timer to return.
> 
ok. I detected this some months ago and I talked with Werner Almesberger
about that. He talked something about del_timer_sync too and other
issues, but I didn't understand that I open a new deadlock case. Now I
learned something new things, thanks. :-)

> The synchornization in this code is really a big mess.

That is one thing which I also detected so I decide to make a new
implementation based on net/ipv6/reassembly.c which also used the
inet_frag api.

I will bring these patches mainline and I hope it will remove the most
of the race condigition. But then it's only solved in net-next branch.

- Alex
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